Promoting release from "Internal test" track to production

In the new Google Play Console (October 2020) you will find it on the right-hand-side menu in:

Release > Testing > Closed/Open Testing > Manage Track > Promote Release > Production

As of this time, Google does allow you to promote an "Internal testing" release to Production.

Under Release on the left, then under Internal testing in the left, click on Internal Testing and then, on the right/main pane click on Promote Release and then choose Closed testing, Open testing or Production.

App releases > Production track click Edit Releases click From Library and choose the APK (by version number) that is in Internal test